Why actors slow down the simulation considerably

Although actors in Gazebo simulation is not simulated as a physical object, why they are slowing down the simulator considerably. The default walking animation with 20 actors reduce the real time factor of the simulator from 0.96 to 0.40 on average. I think this is quite limiting to use swarm of actors in a world. Is there way to speed up the processing?

1 Answer

In this design, to animate each Actor
in the scene, the poses (position and
rotation angles) of each bone in the
skeleton are calculated on the server
side, then updated to the client side
one by one. The client side then uses
the bone poses to transform the mesh
vertices. Although the final rendering
of the transformed mesh with textures
is done on the GPU, this design causes
significant communication overhead for
large crowds. In addition, it slows
down other parts of simulation because
all bone pose calculations are done by
the CPU. As a result, the overall
simulation performance is not
satisfactory. I was only able to get
10 FPS for 16 Actors for real time
simulation (1:1 ratio of simulation
time to real time).

My first attempt to improve the design
was to remove the communication
overhead. To do that, I rewrote the
Actor implementation on the server
side to just send the Actor pose
(output from Menge simulation) at each
time step to the client side. The
client side was also modified to
directly use the Ogre3d library
functions to update the individual
bone poses based on the actor’s pose
and simulation time, “software
skinning” in Ogre3d terms. With this
improvement, we could simulate up to
64 actors at real-time speed and
interactive frame rate. The design
shown in picture below:

Then to further improve the
performance, I implemented “hardware
skinning” in which all the animation
related calculations are done on GPU.
This design, shown in Figure below,
utilizes the GPU vertex shader to
offload vertex transformation
calculations from CPU. The gzclient
code on CPU then only controls the
frame of the bone animation data to be
used at a particular iteration
(simulation time step). This greatly
improves the performance, and allowed
up to 625 characters to be rendered in
real time simulations. The shader
program is written in Cg language,
similar to the
hardwareSkinningFourWeights_vp in
Ogre3D sample code, only slightly
modified to handle more bones.